This invention relates to a method of coding signals from multiple geophysical vibrational energy sources used simultaneously in a geophysical seismic survey so that the seismic signals from each source can be separated in processing, at the same time providing for reduction of harmonic distortion and crossfeeding harmonics from the vibrational sources.
A common seismic survey method employs a single seismic source in conjunction with multiple geophone detector arrays arranged equally spaced along a line from the source. The source is activated a number of times in the same position, in order to improve the signal to noise ratio, and then the source and detector arrays are moved along the same line a short distance and the procedure repeated. This procedure is repeated along the predetermined length of the survey line and other designated survey lines in the area to be surveyed. The recorded data is then processed, usually by a digital computer at a headquarters processing department.
It is now common to use vibrational sources of seismic energy for seismic surveying. Typically, the energy is emitted in the form of a sweep of regularly increasing (upsweep) or decreasing (downsweep) frequency in the seismic frequency range. The vibrations are controlled by a control signal, which can control the frequency and phase of the seismic signals.
When only one seismic source is used, the survey procedure can be very time-consuming. With modern signal processing methods, the most expensive part of the seismic survey process has become the field survey period. This period could be shortened if more than one seismic source could be used simultaneously. However, multiple sources can only be used if some means for distinguishing between signals emanating from the different sources can be provided. Further, the vibrational source generates harmonics which, in certain circumstances, can have an energy approaching or even exceeding the fundamental, and which can crossfeed with signals from other sources, giving misleading results when the signals are processed to separate the signals from each source. In addition, the harmonics are a source of noise and can mask weak reflection signals from deeper layers.
U.S. Pat. No. 3,885,225 to Anstey et al proposes a method to distinguish between multiple sources. Anstey et al is directed to a method and apparatus for broad-line seismic profiling, using several vibrators simultaneously emitting signals. The normal emission frequency bandwidth is divided into several parts which are allocated to individual vibrators in a sequence of separate emissions, in such a way that mutually exclusive frequencies are radiated by the several vibrators at any one time. The detected signals are separated on the basis of frequency to represent the individual signals from each vibrator. However, the frequency limitation on each individual vibrator reduces the sensitivity of the survey. Further, the '225 patent admits that harmonic distortion in the vibrators or their coupling with the ground can impair the capacity of the correlation process to separate the signals from different generators, but makes no attempt to remove or reduce this distortion.
Another method of separating signals from multiple vibratory sources using phase shifting of the signals on different sweeps is disclosed in U.S. Pat. No. 4,715,020 to Landrum. However, the problem of harmonic distortion and crossfeed is not even mentioned in this patent. On the other hand, U.S. Pat. No. 4,042,910 to Rietsch does provide a method of suppressing harmonics using phase shifting, but only for a single vibratory source. Obviously, when there is only one vibrator, crossfeed is not a problem, so this aspect of harmonic suppression is not even discussed by Rietsch.
A recent patent, U.S. Pat. No. 4,823,326 to Ward, does claim a method for producing separate seismic records derived from multiple, concurrently operated vibrational seismic sources, with reduced harmonic distortion. However, the specification only teaches how to achieve this result for two sources, and there is no clue as to how the method could be extended to more than two sources. The selection of suitable initial phase angles for the sources by trial and error, which appears to be what Ward used, becomes almost impossible with larger numbers of sources. For example, when four sources are used, with sixteen sweeps for each source, there are over 1800 possible combinations to be investigated. Further, Ward does not even mention the problem of crossfeed of the harmonics from one source into the signal from another source, and there is no teaching how to solve this problem.